Method of thermochemically cutting ferrous metal



Patented Jan. 22, 1952 METHOD or THERMOCHEMICALLY CUTTING reasons METALHarold R. Fisher, Metuchen, N. .L, assignor to Air Reduction Company,Incorporated, New York, N. Y., a corporation of New York No Drawing.Application April 2, 1951,

, Serial No. 218,900

r Claims. (01. 148--9) This invention relates to the oxygen-cutting offerrous metals and alloys which resist or are immune to the ordinarythermochemical cutting techniques. Examplesof such cut-resistant ferrousmetals and alloys are stainless steel and other alloy steels containinga large percentage of chromium, or chromium and nickel.

Stainless steel, and other alloy steels containing a large percentage ofchromium, are diflicult to cut thermochemically with an oxygen-cuttingtorch because the chromic oxides formed during the cutting operation arevery refractory and do not melt and flow away as molten slag but remainin the kerf and interfere with the cutting operation.

The principal object of this invention is to provide an improvedthermochemical cutting flux and flux cutting method which makes possiblethe cutting of cut-resistant metals including stainless steel and otheralloy steels containing a large percentage of chromium, or nickel andchromium, in a rapid, eflicient and economical way.

I have found that an alloy steel containing an amount of chromium whichwould ordinarily make thermochemical cutting of the alloy steeldifiicult or impossible, can be satisfactorily cut by the use of a fluxin the cutting-oxygen stream comprising an inorganic salt containingsodium and sulphur, such as sodium sulphite (NazSOa),

.oxygen stream atany convenient place so long as it will be present withthe cutting-oxygen in the kerf or at the zone where the thermochemicalaction is taking place. It might even be projected as a separate streaminto the kerf so that it is introduced into the cutting-oxygen stream atthe zone where the thermochemical action is occurring, but preferably itis introduced into the cutting-oxygen stream before the cuttingoxygenreaches the cutting torch.

Any suitable apparatus may be used for introducing the powdered fluxuniformly and at the desired rate into the cutting-oxygen stream, suchas that disclosed in the application of Joseph M. Tyrner, Serial No.696,258, filed September 11, 1946, now Patent No. 2,549,033, issuedApril -17, 1951 and assigned to the assignee of the present application.The apparatus in that application comprises an electrically vibratedfeed hopper for the powdered flux to be introduced into the oxygenstream, a gas-tight casing in which the feed hopper is located andhaving inlet and outlet connections by which the casing can be insertedin the cutting-oxygen line leading to the cutting-torch, and anelectrically vibrated chute within the casing for receiving the powderedflux vibrated out of the hopper and for conducting it to a positionwhere it is sifted into the cutting-oxygen as it leaves the casingthrough the outlet.

The cutting-oxygen stream containing the powdered flux is projected bythe cutting-torch against the high chromium content alloy steel whilethe metal is at kindling temperature and the cutting is effectedthermochemically by the combined action of the cutting-oxygen stream andthe powdered flux. The heating of the metal to its kindling temperaturemay be eiiected by the usual preheating flames of the cutting-torch, orin any other suitable way.

The manner in which the kind of flux material contemplated by theinvention acts to facilitate the cutting of ferrous metals and alloyshaving a high chromium or chromium-nickel content can be explained, itis believed, on the basis of the sodium and sulphur constituents. It isthought that the sulphur-containing anion of the flux attacks the ironand/or nickel of the metal or alloy, dissolving them as complex sulphurcompounds in the molten slag. At the same time, the alkaline sodium ionreacts with or dissolves the chromium to form chromates or chromiteswhich are also carried away in the slag. The state of oxidation of thesulphur compounds in the slag is highest on the surface adjacent to theoxygen stream, and lowest next to the metal, the reducing action ofwhich utilizes some of the oxygen combined with the sulphur. Thussulphur is acting as an oxygen carrier as Well as a slagformingmaterial. The reactions caused by the sulphur-containing anion and thesodium ion prevent the formation of a film of refractory chromic oxidewhich hinders the cutting of stainless steel. Formation of such film isprevented even with steels having a chromium or nickel and chromiumcontent so high that fluxes which do not contain sulphur are ineiiectivefor cutting. The melting point of 'ohromic oxide (CIzOa) is 1999" C.,while the melting point of sodium chromate (NazCROi) is 392 C, accordingto Langes Handbook of Chemistry The low-melting chromates are easilyremoved from the kerf by the cutting-oxygen jet during the cuttingoperation to permit rapid and efficient cutting.

The fluxes of the present invention, containing sodium and sulphur, aremore effective than metal. powders which burn exothermically to increasethe temperature in the, kerf and serve as diiuents and solvents for therefractory materials. They are also more effective than alkaline fluxes,such as sodium bicarbonate, which attack only the chromium and which arenot effective when the chromium content is very high, or when sufficientnickel is present to increase the resistance of tlie work to alkalineattack.

Of the flux materials mentioned, sodium sulphite has been found toproduce the most satisfactory results, especially when cutting caststeels containing a large percentage of chromium. Sodium sulphite istherefore the preferred fluxing agent. No difiiculty is encountered infeed-.- ing it into the cutting-oxygen stream or through thecutting-torch, and when it is used, only slight preheating of theworkpiece is needed to. start a cut, and the thermochemical reactionpasses through the metal very quickly. The cutis easily maintained, butif lost it can be regained easily. The slag produced has a low-meltingpoint, and as the material cools, the slag detaches itself readily fromthe workpiece. Moreover, only a very small drag occurs during thecutting when a sodium sulphite flux is used in the cutting-oxygen streamand a smoother cut face is produced. Little or no static electricitybuilds up in the gas passage.

Sodium sulphite has been found to have fluxing qualities superior toother lmown fluxes when introduced into the cutting-oxygen stream duringthe cutting of both wroughtand cast alloy steels containing a largepercentageof chromium, and when tested against known fluxes, such asiron powder and sodium bicarbonate, in the thermochemical cutting ofcast alloysteels haviing a high chromium content, was foundto. be theonly one of these fluxes tested that eliminated the difficulties presentin the cutting of this kind of alloy steel.

I have found that sodium sulphite is very successful in cuttingaustenitic Cr-Ni stainless steels-alloy steels that containprincipally.chromium, iron and nickel. In addition to the large amountof chromium which resists cutting, the austenitic steels contain nickelwhich, increases the resistance of steel to any-alkaline attack and toany chemical corrosion. The nickel forms a tenacious oxide film on thesteel surface, which film is generally resistant to attack by a kalinematerials. However, with sodium sulphitethe sulphur constituent attacksnickel readily, while the sodium forms sodium chromates whichhavelow-.nelting points and therefore flow easily as explainedhereinbefore, especially under the action of the cutting-oxygen jetSulphur not only attacks iron and nickel, but also aids in movingmechanically any oxides of chromium up to the exposed surface where theyare attacked by sodium. Sodium also acts as a flux on chromium itself.Further, the abrasive action of the flowing stream of sodium sulphitehelps to move the products of chemical reaction.

Extensive experiments have shown that a surprisingly small amount ofsodium sulphite flux is required. for cutting workpieces of variousthicknesses. In cutting practices with adjuvant metallic powde s, forexample, one pound of powder is used for from 15 to 100 cubic feet ofoxygen. With sodium sulphite, however, satisfactory results have beenobtained with as little as one pound of such flux for as high as800-cubic feet of oxygen. And a still smaller ratio of flux to oxygenwill cut properly if the feeding apparatus is designed to feed the fluxsatisfactorily. Good results have been obtained using commerciallyavailable conventional feeders with one pound of sulphite flux for every25 to 800 cubic feet of oxygen; for preferred commercial practice, onepound of flux for every to 450 cubic feet of' oxygen is recommended. Thelatter range permits optimum results with ordinarily availableequipment. XIV-hen comparing sodium sulphite to iron powder, forexample, the amount -i of flux required is reduced from 200% to at least800%, This enables a material reduction in totalcost which is obtainedwithout lessening the efliciency or speed of the cutting operation. Theminimum sodium sulphite flux feeds, in-

- cluding the one pound of flux for 800 cubic feet content, and thelatter includes a 17-49% chromiurn and ii -12% nickel content.

In cutting stainless steel workpieces of different compositions and of2-inch to 10-inch thicknesses, the preferred sodium sulphite feed rangesfrom (1.4 to 2.75 pounds per hour, while conventionalpracticeswith othermetallic powder fluxes, such as ironpowder, call for a range of 1 5 to30 pounds per hour. The following results were obtained in cuttingvarious stainless steel compositions with sodium sulphite fed into acutting-oxygen stream with the apparatus disclosed in the said Tyrner.application:

. Cutting Thickness Flux Oxygen- I Nickel of Rate of Flux Vi orkpieceFeed Feed Ratio Per Cent Per Cent Inches Lbs./hr. OIL/hr GIL/lb 24-26-l922 56- 1. 5 100 6G. 7 22-24 12-15 95 4:1 26. 8 24-26 19-22. 35 4. l110 26. 8 14-16 33 36 4.1 110 26.8 17-19 8-i0 2 1. 5 116. 7 22-24 12-158 1. 5 310 206. 7 17-19 812 8 1. 5 620 413. 3 17-19 8 l2 8 1. 9 750 394.7 18-20 .810 10' 1.0 650 650. 0 18-20- 8.-l0 11 1.0 650 650. 0

The sodium sulphite flux is screened to eliminate coarse particles, Forcommercial use, I have found it preferable to use powdered flux all ofwhich passes through a 40 mesh sieve (0.0165 in. openings) and not morethan 10% by weight passes through a 100 mesh sieve'(0.0059 in.openings). *If any. coarser particles are used, the flux has a tendencyto clog or cake in the apparatus; and if the particles are finer, theflux does not need'feed properly.

Sodium-sulphate, sodium bisulphate, and sodium thiosulphate also canbeused according to the invention in cutting alloy steels having a highchromium content. The'apparent theory of cutting with sodium sulphate,sodium bisulphate, and sodium thiosulphate is believed to be the same asthat for sodium sulphite, as explained hereinbefore. All four fluxes areinorganic salts containing sodium and sulphur, and would therefore beexpected to act in a similar manner. However, the'feedingcharacteristics of such materials are not as good as those of sodiumsulphite, for the former fluxes have a tendency to solidify orclog inthe apparatus.

While particularly useful for cutting steels containing relatively largeamounts of chromium, or chromium and nickel, the sodium-sulphur fluxesof this invention are also useful for cutting other metals. Forinstance, in the case of cast iron, the relatively high carbon contentdoes not oxidize as readily as iron, and thus forms a protectivecovering over the metal. I have found that sodium sulphite, however,greatly accelerates the cutting action of the oxygen stream on castiron, probably because the sulphite attacks the iron underneath thecarbon layer, thereby loosening the carbon and sweeping it out with theslag.

This application is a continuation-in-part of my co-pending applicationSerial No. 90,548, filed April 29, 1949, now abandoned.

I claim:

1. The method of thermochemically cutting alloy steel containing apercentage of chromium which renders said steel difficult to cutthermochemically, which comprises projecting a stream of cutting-oxygenagainst the alloy steel to be out while the metal is at kindlingtemperature and introducing into the cutting-oxygen stream a flux inpowdered form consisting essentially of at least one material selectedfrom the group consisting of sodium sulphite, sodium sulphate, sodiumbisulphate and sodium thiosulphate.

2. The method of thermochemically cutting cast iron containing apercentage of carbon which renders said metal difficult to cutthermochemically, which comprises projecting a stream of cutting-oxygenagainst the metal to be out while the metal is at kindling temperatureand introducing into the cutting-oxygen stream a flux in powdered formconsisting essentially of at least one material selected from the groupconsisting of sodium sulphite, sodium sulphate, sodium bisulphate, andsodium thiosulphate.

3. The method of thermochemically cutting a ferrous metal selected fromthe group consisting of alloy steel and cast iron containing apercentage of chromium and carbon, respectively, which render said metaldifiicult to cut thermochemically, which comprises projecting a streamof cutting-oxygen against the metal to be out while the metal is atkindling temperature and introducing into the cutting-oxygen stream aflux in powdered form consisting essentially of at least one materialselected from the group consisting of sodium sulphite, sodium sulphate,so dium bisulphate, and sodium thiosulphate.

4. The method of thermochemically cutting a ferrous metal selected fromthe group consisting of alloy steel and cast iron containing apercentage of chromium and carbon, respectively, which render saidmetals diflicult to cut thermochemically, which comprises projecting astream of cutting-oxygen against the metal to be cut while the metal isat kindling temperature and introducing into the cutting-oxygen stream aflux in powdered form consisting essentially of sodium sulphite.

5. A method of cutting in accordance with claim 4, said sodium sulphiteflux being introduced into the cutting-oxygen stream in a proportion ofone pound of flux for from 25 to 800 cubic feet of oxygen.

6. A method of cutting in accordance with claim 4, said flux consistingof finely-divided powder all of which will pass a 40 mesh sieve and notmore than 10% by weight of which will pass through a mesh sieve.

7. A method of thermochemically cutting stainless steels comprisingprojecting a stream of cutting-oxygen against a surface portion of astainless steel body while said surface portion is at or above thekindling temperature, and introducing sodium sulphite into the zone ofthermochemical action between said cutting-oxygen stream and saidstainless steel body whereby the cutting is effected thermochemically bythe combined action of the cutting-oxygen stream and the sodiumsulphite.

HAROLD R. FISHER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 249,393 Patten Nov. 8, 1881341,784 Whitcomb et a1. May 11, 1886 650,124 Coleman May 22, 1900750,512 Wherry Jan. 26, 1904 968,350 Harrison Aug. 23, 1910 1,550,280Post -1 Aug. 18, 1925 2,144,208 Van Meter Jan. 17, 1939 2,321,309 MillerJune 8, 1943 2,415,815 Deming Feb. 18, 1947 2,451,422 Wagner Oct. 12,1948 2,454,325 Linnert Nov. 23, 1948 FOREIGN PATENTS Number Country Date568,397 Great Britain Apr. 3, 1945 OTHER REFERENCES Mellor:Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 11,1931, pages -182; pub. by Longmans, Green and Co., New York.

Metals Handbook, 1939 edition, pages 930-935; pub. by Amer. Soc. forMetals, Cleveland, Ohio.

Iron Age, vol. 156, August 9, 1945, page 61.

Metals Handbook, 1948 edition, pages 383, 384; pub. by Amer. Soc. forMetals, Cleveland, Ohio.

1. THE METHOD OF THERMOCHEMICALLY CUTTING ALLOY STEEL CONTAINING APERCENTAGE OF CHROMIUM WHICH RENDERS SAID STEEL DIFFICULT TO CUTTHERMOCHEMICALLY, WHICH COMPRISES PROJECTING A STREAM OF CUTTING-OXYGENAGAINST THE ALLOY STEEL TO BE CUT WHILE THE METAL IS AT KINDLINGTEMPERATURE AND INTRODUCING INTO THE CUTTING-OXYGEN STREAM A FLUX INPOWDERED FORM CONSISTING ESSENTIALLY OF AT LEAST ONE MATERIAL SELECTEDFROM THE GROUP CONSISTING OF SODIUM SULPHITE, SODIUM SULPHATE, SODIUMBISULPHATE AND SODIUM THIOSULPHATE.